1. Field of the Invention
The present invention relates to network printing systems, and in particular, to a method and apparatus for performing digital half-tone screen calibrations.
2. Description of the Related Art
Network printing systems generally comprise an assemblage of different printers, client computers, servers, and other components connected over a network. A print job is assembled at a client computer and transmitted over the network to a server linked to a variety of printers. Although these printers have historically been used to reproduce text, they are increasingly being used to reproduce graphic and image files, which have significant grayscale content.
Digital printers with grayscale reproduction capabilities use half tone screens with or without dot density control (contone) to reproduce the grayscales. Due to differences in dot gain characteristics, types of screens, and contone designs, grayscale characteristics can vary widely from printer to printer. Consequently, when graphic and image files generated for particular grayscale characteristics are sent to a printer with different characteristics, the appearance of the printed output may be substantially different. Such variations are more pronounced among bi-level printers, because dots are not as easily controlled as they are with contone printers.
These variations often result in visually unacceptable results, because proper rendering of grayscale information is critical to the reproduction of graphical and image files. This problem can be avoided by imposing a print job routing scheme that associates particular graphical and image files with a particular printer in the network, but that requires storage, enforcement, and management of these associations, and is antithetical to the flexible and distributed nature of a network computing system. Also, by confining the printing of a grayscale image to a particular printer, this approach would not allow an individual to adjust the image quality of a desired print on a local printer before transferring the print file to a higher quality or higher capacity printer for the final product. There is therefore a need for a system that allows consistent grayscale printing among a variety of printers in a network, each with different grayscale printing characteristics.
To address the requirements described above, the present invention discloses a method, apparatus, and article of manufacture for the calibration of a halftone screen of a first printer to simulate the grayscale characteristics of second printer. The method comprises the steps of characterizing the grayscale printing characteristics of the first and the second printer and generating a transform between the grayscale printing characteristics of the first printer and the grayscale printing characteristics of the second printer. In one embodiment, the characteristics of the first and the second printer are generated by printing patches of gray levels with the first and the second printer, with each gray level corresponding to a gray level input command, measuring the printed gray level of the patches, normalizing and smoothing the gray level measurements, and defining functions which characterize the relationship between the printed gray level and the gray level input command. These functions are used to derive a transform which can be used to configure the first printer to emulate the grayscale printing characteristics of the second printer. The apparatus comprises a first and a second printer for printing grayscales, a measurement means such as a reflectometer or densitometer to measure the printed grayscales, and means, such as a processor, for determining a transform between the grayscale printing characteristics of the first printer and the second printer. In this context, the xe2x80x9csecond printerxe2x80x9d may not be a physical device, but instead represent an ideal or desired printer with known characteristics.